Altus 9 speed derailleur downshift problem
 

Upshifting relatively fine, but downshifting balky/delayed.... Der is maybe 3 yrs old. Adjusted a number of times from set screws to barrels... no joy....

Suggestions?

Define "upshift" and "downshift". If "upshift" isn't your derailleur going to a smaller rear cog, you have a cable friction problem
Limit setscrews WILL NOT fix your problem. Don't touch them

Check th spring - it can get rusty. If so, blast it with some WD40 and work it back and forth.

Sorry, "downshift" meaning going from large cassette ring to small....

That I can do! If no go... use dry lube on the cables (per previous post)?

If the cables are as old as the derailleur, and have significant use, they may be due for a replacement, especially considering the symptoms.

You've got it backwards.

A downshift goes to a larger cog/lower gear.
Cable pulls the RDER through positive force.
An upshift relies solely on RDER spring tension trying to over ride the friction of your cable/housing.
Clean & lube your cables.
The rear "loop" is especially susceptible to road grime.

Thanks, Bill Kapaun, you are correct.... Swear, sometimes, that I was born a breach baby....

Clean and lubed at the cable housing, last night, and got better... gonna wd40 the spring tonight....

I'd replace the housing too.

Often, replacing the rear housing "loop" can have great effect.
Because of the sharp bend (& grit) the cable can wear kind of a "groove" which will increase friction.

Eventually, the straighter/less curved runs need to be replaced, but you can probably do 2-3 loops first.

Ease kind of depends on how frayed/kinked the cable end is. You have to insert it into the new housing without disturbing/losing strands, else new cable.
Of course, a new cable isn't a major job if you have a DECENT cable cutter/crimper.

I always replace both. The housing gets "worn in" by the inner wire (which is the cause for much of the "cable stretch" we see) and may not work as well with a new, different inner one. Plus it gets rid of any rust, dirt or old, hardened lubricant. Since I don't have any under-the-tape housing runs it is easy to do.

WD, cleaning and lubing worked!

Thanks :)